1. Field of the Invention
The present invention relates to a method for time-division multiplexing adapted for SDH (synchronous digital hierarchy).
2. Description of the Related Art
The SDH, which is the basis of a broadband ISDN, is a hierarchy achieved through global unification of communication speed series by grouping a multiplicity of 64-kbps signals into some stages of various rates. More specifically, the SDH prescribes the interface to effectively multiplex a variety of high speed services and also the existing low speed services, and it was standardized in conformity with CCITT (present ITU-T) in November, 1988. The features of such multiplexing include that a fundamental transmission rate is set to 155.52 Mbps (termed STM-1 level) on premise that the network frequency is in synchronism, and a transmission rate of N×155.52 Mbps, which is an integral multiple of the fundamental rate, is used. For example, there are prescribed as N=1, 4 and 16, i.e., 155.52 Mbps (STM-1), 622.08 Mbps (STM-4), and 2.48832 Gbps (STM-16). The maximum SDH multiplexing rate standardized at present is 10 Gbps. However, with the recent rapid increase of the amount of the information in the latest data communications, it is highly demanded to realize higher rate communications in the basic transmitting equipment. For example, a transmission rate of 40 Gbps can be achieved in practical use by time-division multiplexing 16-channel SDH signals each of 2.4 Gbps (STM-16).
A frame structure at STM-1 is composed of a two-dimensional byte array with 9 rows and 270 columns. Bytes in the first 9 rows and 9 columns (except 4th row), are termed a section overhead (SOH) including frame sync signal, maintenance information, state monitor and other information. The first 4th-row byte is termed an AU-pointer which indicates the top position of main information. The 9 rows and 261 columns except the SOH and the AU-pointer are termed a payload where main information is stored.
In an SDH frame overhead, there are prepared two patterns termed an A1 byte (“11110110”) and an A2 byte (“00101000”) for synchronizing the frame. In an SDH frame of 2.4 Gbps, 48 A1 bytes and 48 A2 bytes are arrayed successively. And a channel identification byte termed a J0 byte is prepared next to the A1 and A2 bytes.
Now a consideration will be given on an exemplary case of producing 40-Gbps signals by time-division multiplexing such 2.4-Gbps SDH frames of 16 channels simply by bit interleave while disregarding the respective frame phases of the channels. In this case, there exists a possibility that the entire SDH frames of 16 channels may be multiplexed in the same phase, whereby in the A1-byte and A2-byte portions of the SDH overhead, 64 successive “1” bits are arrayed in the A1 byte multiplexed area, and 80 successive “0” bits are arrayed in the A2 byte multiplexed area (1536 bytes=96×16 channels). Meanwhile the mark rate in the A1 byte area is 0.75, and the mark rate in the A2 byte area is 0.25 (which will be described in detail later).
Such long successions of the same code or such deviations of the mark rate may cause harmful influence on both a transmitter and a receiver in any extremely fast transmission performed at a high transmission rate of 40 Gpbs or so. For example, in an optical receiving module, its clock reproduction capability is deteriorated essentially by long successions of the same code or deviations of the mark rate.
It is therefore an object of the present invention to provide an improved time-division multiplexing method which is capable of reducing the number of successive same code and diminishing the deviation of the mark rate.